1. Technical Field
This invention relates to a method for making printed circuit boards and the like. More particularly, it involves a method of making circuit boards wherein conductor material is electrolessly deposited onto seeded areas on a dielectric substrate.
2. Background Art
In the manufacture of printed circuit boards and the like, a conductive circuit pattern is provided on at least one surface of a dielectric substrate. The conductor pattern can be formed on the substrate using a variety of known techniques. One of the better known techniques includes the electroless direct bond (EDB) technique where copper is electrolessly plated directly onto the surface of the substrate in the desired pattern. The conductor pattern may include conductor lines on the surface of the substrate as well as in holes (often referred to as plated through holes) which connect one or more conductive layers together. Since the dielectric substrate is non-conductive, it is generally necessary to seed or catalyze the substrate prior to deposition of the metal onto the substrate. Among the more widely employed procedures for seeding the substrate is to deposit colloidal particles of palladium (Pd) and tin (Sn) on the substrate. A copper metallic coating can then be applied to the seeded areas by way of well known electroless deposition techniques.
U.S. Pat. No. 4,448,804 to Amelio et al, which is assigned to the assignee of the present invention, discloses a particularly advantageous technique for making printed circuit boards and the like. Briefly, that patent discloses the steps of coating a previously seeded substrate with a maskant such as a photoresist. The photoresist is then developed leaving exposed those seeded areas which correspond to the desired conductor pattern. Copper is then electrolessly plated onto the substrate and the remaining photoresist is removed.
While the process noted above has provided extremely satisfactory results, electrical shorts between the conductor lines on the substrate have been occasionally experienced. Of course, such problems are to be avoided if at all possible.